What can go wrong? Almost anything, so this is something you would want to avoid except in a maintenance window. Although, in theory, it can be done anytime. Ideally, you don't want to connect a stack cable between two live stack members.
First, insure all links you wish to continue to use have been migrated.
Then the "safe way" power-off whole stack, remove stack members 4 and 7, reconnect stack cables bypass the removed stack members. Then power-on stack memer desired to be stack master, followed by other remaining stack members.
Otherwise, second, determine if either of the switches you wish to remove are the stack master. If one is, you'll take at least a brief L3 hit when your remove the current stack master, and depending on your configuration, the stack's MAC will change.
Third, insure you have a "full duplex" stack.
Fourth, power off one of the switches you wish to remove but not the one that is current stack master.
Fifth, restore stack cabling (i.e. jump pass powered off switch) and insure it's once again "full duplex"
Sixth, power off other stack member.
Seventh, bypass removed stack member stack cabling and insure stack is "full duplex".
Lastly (optional), remove config information for removed stack members.
